Cooking vessels are generally produced from a deep-drawn aluminum sheet.
During heating, the surface of the bottom of the vessel, in contact with a cooking plate or a burner, is exposed to a temperature markedly higher than the inside surface of the said bottom.
As a result, thermal expansion gives rise to a convexity of the bottom of the vessel.
In order to prevent this detrimental phenomenon, the bottom of the vessel is given a slight concavity so that, during heating, this bottom becomes substantially planar under the effect of the expansion. On cooling, the bottom theoretically resumes its concavity.
However, experience shows that, after a certain number of repeated cycles of heating and cooling, the concavity of the bottom of the vessel decreases, then becomes zero and eventually the bottom becomes convex, the vessel thus becoming virtually unusable.
This result is explained by the properties of aluminum.
During repeated cycles of heating and cooling, the elastic region is exceeded, resulting in irreversible deformation.